flected;
but it is not so well known that if to this ray, which has already been
reflected at a certain angle, a second mirror with plane inclined is
presented, there are certain positions jn which the ray will be no
longer reflected by the second mirror. The light has acquired by its
first reflection a profound modification, which is designated by the
name of polarization by reflection.
In
traversing certain crystals the light is subjected to the same
changes; that is to say, the rays emerging from the crystal are no
longer reflected when they fall at a certain angle upon a plane mirror;
and they have become completely powerless to traverse certain crystals,
otherwise perfectly trans-lucid, when the latter are presented to them
according to a determinate direction. The phenomenon thus presented is
polarization by refraction.
Double
refraction and polarization are qualities of crystals which are most
intimately connected; and the combinations of these two manifestations
produce magnificent phenomena of colouring, unattainable by substances
producing simple refraction. It is very easy, by aid of a polariscope,
to be assured upon the instant whether a precious stone possesses or
not the power of double refraction.
Dichroism, polychroism, asteria.—The phenomena designated by these expressions, and which give a magical beauty to certain precious stones,
